Precision temperature forcing systems (PTFS) provide a low-cost means to thermally test a device under test (DUT). The thermal head of a PTFS is designed for coplanar positioning of the bottom edges of its thermal cap and glass shroud. This usually involves sealing the thermal cap and shroud directly to the host printed board (PB) of the DUT.
A thermal cap attached to the air flow nozzle of the PTFS thermal head directs temperature controlled air directly onto the DUT and then out through its vent holes into the shroud area and eventually out the vent holes of the thermal head. The thermal cap is intended to minimize the air volume directly around the DUT and reduce the air flow rate necessary to force the target temperatures.
However, standard conductive or nonconductive rubber thermal caps can only accommodate a limited range of component sizes. When a DUT or its test socket is too large for a standard thermal cap and the thermal cap cannot be retracted far enough to accommodate the height of the DUT or its test socket, the thermal cap can be omitted. However, once the thermal cap is omitted the entire shroud air volume must be forced to the target temperatures. The extra thermal load slows down temperature transition times and also requires higher air flow rates which can cause condensation and icing issues at extended cold temperatures.
In an attempt to solve this problem, the PB area around the large socket is built up with a suitable material, such as conductive or nonconductive silicone foam rubber, to raise the shroud footprint up to the top of the test socket so that a thermal cap can be sealed against the top of the DUT or its test socket. With this configuration, very little of the forced air reaches the exposed area of the DUT. The rest of the forced air is deflected off the component carrier of the test socket or flows directly out the thermal cap vents without ever reaching the DUT. In addition, only a small portion of the DUT body surface is exposed to the forced air. The rest is insulated by the component carrier and contact base of the test socket. This results in a rather poor thermal transfer between the forced air and the DUT.